In conventional color electrophotography, a series of electrostatic images are created on an image member. For example, a charged photoconductive drum is imagewise exposed with an electronic flash, an electronic printhead or optical scanner. The electrostatic images are toned with different colors to create a series of different color toner images. The toner images are then transferred in registration to a receiving sheet which is repeatedly presented to the image member. Conventionally, the receiving sheet is fixed to the periphery of a transfer roller which engages the image member and rotates the receiving sheet repeatedly into transfer relation with the images as they are presented.
As dry toners are successfully made smaller, higher quality images can be formed on the image member.
Transfer of images made up of extremely fine toner particles is more difficult than transfer of more coarse toner. Ordinary electrostatic transfer has not proven as effective as has transfer involving a combination of pressure and heat.
Pressures in excess of 40 pounds per square inch, preferably pressures substantially in excess of 100 pounds per square inch, have been found to be effective in transferring small particle toners which have been heated to either their softening or sintering point. Although such a transfer process is effective with ordinary paper receivers, it is especially effective with receivers having a surface coating of a heat softenable thermoplastic. The heat softenable thermoplastic is heated to its softening point which in turn heats the toner in the nip between the receiving sheet and the toner image. The toner softens or sinters and portions of it embed in the thermoplastic material, while other portions cling to the toner so embedded. This process is most effective with much higher pressures than those used in conventional color electrophotography.
A principle limiting aspect of such systems is the ability to transfer the three or four color images in extremely accurate registration. Such registration requires that a receiving sheet carried by the transfer roller accurately present itself to each consecutive image on the image member. U.S. Pat. No. 4,872,037 shows an approach to precision in systems in which a photoconductive drum and a transfer roller are each independently driven by servo motors with the location of the transfer drum periodically monitored. A computer controls the speed of the transfer roller to assure registration of the images. With such an approach, it may be possible to accurately superimpose images in systems in which no or a light pressure is required between the image bearing member and the receiving sheet. If an independently driven drum and roller were used with substantial pressures as required in the transfer process described above, substantial wear to the image member, generally having a photoconductive surface, would result.
A solution to this problem is to allow the transfer roller to be driven by the image member. With such an approach, excessive wear between the members is not a problem. However, as multiple copies are made, registration between the transfer roller and image member has a tendency to drift. Such drift is especially prevalent if heat is used to assist transfer because of expansion of the transfer roller. Variations in thickness of the receiving sheet also can cause drift. Such drift will cause the means for securing the receiving sheet to the transfer roller, for example vacuum holes or gripping fingers, to face a different portion of the image member. If the image member is discontinuous and therefore requires that the images be formed on a particular portion, such drift is not acceptable.